In a typical nuclear reactor power plant, a nuclear reactor vessel is used to generate heat for the production of steam and electricity. In one such design, the reactor vessel is a pressure vessel which encloses the core of nuclear fuel and coolant, typically borated water. As a means for generating data relating to the operating conditions within the pressure vessel, instrumentation devices are introduced to the nuclear core through ports or penetrations in the vessel. Some of these penetrations are provided for through the reactor vessel closure head. The closure head may also include penetrations for drive mechanisms of control rods, used to regulate the rate of nuclear reactions which take place within the core, and in turn control the power output of the plant.
Although these ports are mechanically sealed to prevent the inadvertent leakage of coolant from the reactor vessel, the operating conditions of a nuclear reactor pressure vessel require additional safeguards. Typically, the pressure vessel maintains the coolant therein at an internal pressure of about 15 MPa (2250 psi) and a temperature of about 315.degree. C. (600.degree. F.). Because of such a large internal pressure, reactor coolant may leak from the mechanical joint of these penetrations, or when control rods are withdrawn.
The coolant within the reactor vessel is slightly acidic and highly corrosive due to the presence of boric acid which is dissolved within the coolant. Boric acid is a neutron absorber used as a variable reactivity control over the long-term operation of the plant. Even though there are regulatory limits on the allowable amount of coolant which may be emitted from the reactor vessel, components on the exterior of, and in close proximity to, the reactor vessel head need to be periodically inspected to determine if coolant is being emitted.
Since an operating nuclear reactor generates an irradiated environment, the inspection and/or maintenance of the reactor vessel head area is typically conducted at times when the reactor is shut down for normal inspection or maintenance procedures, such as refueling of the core. A usual telltale sign of the presence of a leak in this area is white boric acid crystal deposits on the reactor vessel head. Any of several methods for determining the presence of a possible leak source may be used, but verification of the existence of a leak, estimations of its size, and the identification of its location is best done visually. Generally, the control rod drive mechanisms and instrumentation ports are enclosed by a cooling shroud. This shroud provides protection for the drive mechanisms, as well as a means for directing the flow of air around the ports for natural circulation cooling of the ports and drive mechanisms. This can make it even more difficult to visually detect the presence of borated coolant in this area.